1. Field of the Invention
This invention relates to a light beam scanning system using a rotating multi-face mirror for application in a laser beam recording device, and more particularly to a light beam scanning system which is free from the jitter in the direction of the scanning lines which ordinarily occurs in a scanning system using a rotating multi-face mirror because of imprecision in manufacture (imprecision in the angle between adjacent mirror faces), fluctuations in the speed of rotation and mechanical vibration.
2. Description of the Prior Art
Numerous recording devices employing a laser beam as the scanning beam have been developed. In each of these an important role is played by the deflector used in the laser beam scanning system. Among the various types of known deflectors some employ an electro-optical or acoustical-optical effect while others employ a vibrating mirror in conjunction with a galvanometer or a rotating multi-face mirror. Systems employing a rotating multi-face mirror as the deflector are advantageous in that the angle of deflection is large, the resolution is high and spectral dispersion is nil. On the other hand, they are disadvantageous in that repeated scanning at precise intervals is difficult or impossible to attain since imprecisions in the rotating multiface mirror (mainly imprecision in the angles between adjacent mirror faces) and variations in the speed of rotation of the motor driving the multi-face mirror give rise to irregularities in the starting points of the scanning lines.
Take, for example, the problem arising from angular error between adjacent faces of the multi-face mirror. In a case where a mirror having 24 faces is used and 80% of the scanning line is taken as the effective scanning line, then if the effective scanning line is divided into 1500 dots, the angle of rotation of the multi-face mirror corresponding to a single dot will be: 360.degree./24 .times. 0.8 .times. 1/1500 = 0.008.degree. = 28.8 inches.
Thus an error in the angle between adjacent faces of the multi-face mirror of over 28.8 inches will cause the dots in adjacent scanning lines to be out of phase with each other by an interval corresponding to one or more dots. Where the image produced by scanning is required to have a high degree of resolution, the displacement among the scanning lines must be kept within a small fraction of a dot and an image formed by scanning lines having a displacement of as much as one dot can by no means be called satisfactory. This means, therefore, that the angular error between adjacent mirror faces must be kept within a few seconds.
Manufacture of a multi-face mirror possessing this degree of precision not only requires a high degree of skill but also is very expensive.
Let us next examine the effect of fluctuation in motor speed. If the same mirror having 24 faces is rotated at 3,600 rpm, the scanning frequency becomes 1.44 KHz and the scanning time per scanning line becomes 694.4 .mu.s. As only 80% of the entire scanning line is used, the effective scanning time per line is 555.6 .mu.s and the scanning time per dot is: EQU 555.6 .mu.s/1,500 .apprxeq. 0.37 .mu.s
As the rotating speed of the motor will not be subject to high frequency fluctuations of 1 KHz or more, the fluctuations which occur in the course of a single scanning line, that is, within 694.4 .mu.s, can be ignored. However, when these fluctuations accumulate, they can amount to a large low frequency fluctuation on the order of several to several tens of Hz. Actual measurements show that this fluctuation gives rise to a time error of several .mu.s or more as reckoned against an ideal constant speed of rotation. If the time required to scan the equivalent of a single dot is 0.37 .mu.s, an error of this magnitude will result in a shift from the ideal image of ten to twenty dots or even more. As a consequence, it is nesessary to employ a high-performance motor controlled by a complex feed back system in order to hold the shift due to fluctuation in the speed of rotation of the motor to a small fraction of a dot.
As a system for eliminating jitter in the direction of scanning without improving the precision of the rotating multi-face mirror or the motor, the inventors previously proposed the invention described in their Japanese Patent application No. 120478/74. The system according to this previous invention applies to a laser beam recording device wherein the scanning lines are formed by deflecting the light beam by means of a rotating multi-face mirror the rotation of which is not synchronized with the video clock. A light detector is provided at a point within the scanning line immediately preceding the effective scanning line and the electrical signal obtained from the light detector is used as a synchronizing signal. Starting from the termination of this synchronizing signal, basic clock pulses having a frequency n (a positive integer) times that of the unsynchronized video clock are generated. The video clock pulses are obtained by dividing this signal by 1/n thus reducing jitter to as little as 1/n.
In such a system, if it should be desired to use a high-frequency video clock of a frequency of 10 MHz or more, the frequency of the basic clock would have to be 100 MHz. At such frequencies the system therefore becomes impractical since very few circuit elements are capable of responding to such frequencies.